As a thin, high image quality, and low power consumption display device, conventionally, an organic EL (Electro Luminescence) display is known. In the organic EL display, a plurality of pixel circuits, each including an organic EL element which is a current-driven self-luminous type display element and a driving transistor for driving the organic EL element, are arranged in a matrix form.
Methods for controlling the amount of current flowing through a current-driven type display element, such as an organic EL element, are broadly classified into a constant-current type control method (or a current specified type drive method) in which a current to flow through the display element is controlled based on a current flowing through a data signal line; and a constant-voltage type control method (or a voltage specified type drive method) in which a current to flow through the display element is controlled based on a voltage applied to a data signal line. When an organic EL display is allowed to operate by the constant-voltage type control method, there is a need to compensate for variations in the threshold voltage of a driving transistor and a reduction in current (reduction in luminance) resulting from an increase in resistance caused by deterioration over time of an organic EL element. On the other hand, in the constant-current type control method, since the current value of a data signal is controlled such that a constant current flows through an organic EL element irrespective of the above-described threshold voltage and internal resistance of the organic EL element, normally, the above-described compensation is not required. However, with the constant-current type control method, the numbers of driving transistors and wiring lines increase over the constant-voltage type control method, reducing the aperture ratio. Hence, the constant-voltage type control method is widely adopted.
For a pixel circuit that performs the above-described compensation operation in a configuration adopting the constant-voltage type control method, various types of configurations are conventionally known. Japanese Patent Application Laid-Open No. 2006-215275 describes a pixel circuit 80 shown in FIG. 28. The pixel circuit 80 includes TFTs (Thin Film Transistors) 81 to 85, a capacitor 86, and an organic EL element 87. When writing is performed to the pixel circuit 80, first, the TFTs 82 and 84 are placed in an on state, by which the gate-source voltage of the TFT 85 (driving transistor) is initialized. Then, the TFT 84 and the TFT 83 are placed in an off state in turn, by which a threshold voltage of the TFT 85 is held in the capacitor 86. Then, a data potential is applied to a data line DTL and the TFT 81 is placed in an on state. By controlling the TFTs in this manner, variations in the threshold voltage of the TFT 85 and (a reduction in current resulting from) an increase in resistance caused by deterioration over time of the organic EL element 87 can be compensated for.
The pixel circuit 80 is connected to the data line DTL, four control lines WSL, AZL1, AZL2, and DSL, and three power lines (a Vofs wiring line, a Vcc wiring line, and a Vss wiring line). In general, the larger the number of wiring lines (particularly, control lines) connected to the pixel circuit, the more complicated the circuit becomes, increasing manufacturing costs. In view of this, Japanese Patent Application Laid-Open No. 2006-215275 describes the pixel circuit in which the source terminal of the TFT 82 or the TFT 84 is connected to the control line WSL. Japanese Patent Application Laid-Open No. 2007-316453 describes a pixel circuit in which the gate terminal of the TFT 82 is connected to a control line for a preceding row. By thus using a control line and a power line in a shared manner, the number of wiring lines can be reduced.
Japanese Patent Application Laid-Open No. 2007-310311 describes a pixel circuit 90 shown in FIG. 29. The pixel circuit 90 includes a TFT 91, a TFT 92, a capacitor 93, and an organic EL element 94. When writing is performed to the pixel circuit 90, first, the TFT 91 is placed in an on state. Then, an initialization potential is applied to a power line DSL, by which the initialization potential is provided to the anode terminal of the organic EL element 94. Then, by applying a power supply potential to the power line DSL, a threshold voltage of the TFT 92 (driving transistor) is held in the capacitor 93. Then, a data potential is applied to a data line DTL. By thus providing an initialization potential from the power line, variations in the threshold voltage of the TFT 92 can be compensated for with a small number of elements.
In addition, Japanese Patent Application Laid-Open No. 2007-148129 describes a pixel circuit in which an initialization potential is provided from a power line and a reference potential is provided from a data line. Furthermore, Japanese Patent Application Laid-Open No. 2008-33193 describes a pixel circuit that performs compensation operation (operation for compensating for variations in threshold voltage) during a plurality of horizontal periods before performing writing. Moreover, Japanese Patent Application Laid-Open No. 2009-237041 describes a display device in which a threshold voltage variation correction process is performed on a plurality of lines at one time and the scanning order for writing of a plurality of lines on which a variation correction process is performed simultaneously is reversed every field (frame).